The present invention generally relates to digital modulation techniques for land mobile radio systems, and, more particularly, to a method and apparatus for generating a continuous-phase frequency shift keyed (CPFSK) signal by the quadrature modulation of a radio frequency (RF) carrier with filtered, digital data using entirely digital methods.
CPFSK is a subset FSK in which the abrupt spectral transients generated by switching from one frequency to another in FSK are avoided by modulating the frequency of a single oscillator by the information bearing signal. Several constant-envelope CPFSK digital modulation techniques are known which provide spectrally efficient modulation for mobile radio system applications. Such techniques include Gaussian minimum shift keying (GMSK), tamed FM (TFM), and generalized tamed FM (GTFM). For any of these forms of constant-envelope digital modulation, coherent or non-coherent detection methods may be utilized. Although non-coherent detection methods are inherently less complex than coherent detection methods, non-coherent techniques exhibit inferior performance when utilized in mobile radio systems where Gaussian noise is additive over the radio channel, and where multipath effects cause intersymbol interference.
Employing coherent demodulation necessarily implies that some type of carrier recovery mechanism be made available in the receiver. Carrier recovery techniques for constant envelope coherent modulation methods fall into two broad classifications: carrier recovery methods for `continuous` data transmissions; and carrier phase estimation methods for `bursted` data transmissions. Both types of carrier recovery techniques require that the transmitter carrier frequency `f.sub.c ` and the transmitter modulation index `h` (i.e., 2 times the peak deviation divided by the bit rate) be maintained invariant over time, temperture, and power levels.
For continuous data transmission, carrier recovery is usually achieved by an effective squaring operation which permits a carrier reference signal to be obtained directly from the received signal. For all the aforementioned modulation techniques which employ a modulation index of h=0.5, the squaring operation doubles the modulation index. The resultant signal exhibits spectral components at the carrier frequency f.sub.c plus-or-minus one-fourth the bit rate. Precise control of the modulation index is necessary, such that a viable carrier component will exist after the squaring operation. Examples of carrier recovery methods employing this technique include Costas loops, squaring loops, and various open loops.
For bursted data transmission, carrier recovery can be achieved by estimating the carrier phase from the received signal. The estimation is performed by correlating a local replica of a synchronization word with the identical sync word which has been embedded into each transmission burst. Bursted data transmission is preferred over continuous modulation for very high data rate (e.g., 250 kilobits-per-second) mobile radio systems, since a similar sync correlation operation is required in the bursted data carrier recovery process to adaptively equalize the channel to compensate for multipath effects.
The required tolerance on the modulation index for bursted data transmission at h=0.5 is given by the relationship: EQU Tolerance (.+-.)=Y/.pi.X
where Yis the maximum phase offset allowable at the transmitter (in radians), and X is the number of bits in the data burst. For example, if Y=.pi./4 radians and X=58 bits, then the tolerance on the modulation index h=0.5 would be .+-.0.4%. However, recent digital cellular system specifications require the maximum r.m.s. phase error to be 5 degrees (0.087 radians). Hence, using the same number of bits in the data burst, the modulation index must be h.+-.0.5.+-.0.05%. Needless to say, this is an extremely tight tolerance requirement.
Several methods are known for controlling the modulation index of a constant-envelope signal. One method utilizes a standard FM modulator with its deviation controlled through the use of a feedback loop. The feedback loop may incorporate a phase-locked loop, a discriminator for calibration purposes, and/or a deviation error detector with a modulation canceller. However, the use of a feedback loop in whatever form given above is presently only capable of controlling the modulation index to an accuracy of .+-.2%.
A second known method for controlling the modulation index for a constant-envelope signal includes the use of a serial minimum shift keying (MSK) transmitter consisting of a binary phase shift keying (BPSK) modulator and a precise bandpass filter. Such a method is only suitable for unfiltered MSK, since unfiltered MSK corresponds to linear modulation in the quadrature paths. Filtered MSK, however, does not have this property.
A third known method for transmitting a constant-envelope CPFSK signal having a controlled modulation index is to use an analog quadrature modulator to modulate an RF carrier. This method, while capable of adjusting the modulation index to within the tolerance necessary for a bursted communications system, nevertheless suffers from a number of disadvantages, i.e., the requirement of costly high-tolerance parts, frequent manual adjustments, excessive parts count, excessive current drain, etc. In order for an analog modulator to maintain amplitude balance, phase accuracy, and carrier leakage suppression within specification over all possible operating conditions at h=0.5, the modulation index tolerance is typically no better than .+-.0.5%.